1. Field of the Invention
The present invention relates to wireless local area networks (WLANs), and in particular, to a seamless handoff method in the WLAN.
2. Description of the Related Art
A wireless local area network (WLAN) is a type of local area network (LAN) that uses high frequency radio waves rather than wires to communicate between terminals (stations). The WLAN is practical to use in an environment where wire distribution is not allowed or the deployment of the network changes frequently. The WLAN uses a random access technique for media access to allow the stations to competitively transmit data. Since, in the random access scheme, data loss occurs due to the collisions of packets on the radio media and it is difficult to detect the data collisions at the transmitter by nature of the wireless media, a collision avoidance technique is required. Generally, the WLAN adopts carrier sense multiple access/collision avoidance (CSMA/CA), as the collision avoidance scheme, in which collisions or errors can be indirectly monitored during a basic procedure of the data transmission from the transmitter to the receiver. If the transmitter does not receive an acknowledgement of the transmitted data, it retransmits the data after a predetermined time lapses.
Typically, the WLAN has a plurality of Basic Service Sets (BSSs), with each BSS having an Access Point (AP) connected to a wired network, and a plurality of wireless stations (STAs).
Communication within the BSS is provided through the AP such that the STAs are required to associate with an AP in order to access to the network. Thus, when deciding to establish an association with the network, the STA first searches for an AP to be associated with before transmitting data. During the course of data transmission after the association, the STA may roam out of the service area of the current AP. In this case, since the APs use different channels to prevent from interference with each other, the STA must release the association with the current AP and establish a new association with a new AP, which is called a “handoff” in wireless communication terminology. Typically, the BSSs have coverage areas overlapping each other such that the STAs can roam from one area to another within the WLAN and maintain communication anywhere in the WLAN.
Handoff can occur when the STA roams within the BSS or moves into range of other BSSs. Handoff delay is a significant issue for real time multimedia services such as voice-over-IP, video teleconference, and the like.
FIG. 1 shows a typical WLAN structure. In FIG. 1, three access points 11, 12, and 13 are connected to the Internet 17 through a gateway 15 and their coverage areas overlap with each other. The access points 11, 12, and 13 are assigned different radio channels, i.e., respective channel 1, channel 6 and channel 11. In this example, AP2 12 communicates with STA 10 through the channel 6.
When STA 10 moves from service area (B) of the AP2 12 to service area (A or C) of the AP1 11 or AP3 13 while communicating with the AP2 12, handoff occurs. Typically, the handoff procedure comprises scanning, authentication, and association processes. Scanning is a process that attempts to discover the existence of APs and to gather information on the APs. There are two ways for a mobile node to scan for existing APs, active scanning and passive scanning.
For passive scanning, the STA listens for beacon frames, which are periodically transmitted by APs. For active scanning, the STA sends out a probe frame on specific channel to solicit response frames from the APs.
In the passive scanning technique, it typically takes more than 300 msec for receiving the beacon frames from the APs and obtaining information on the APs from the beacon frames. Also, in the active scanning technique, it typically takes more than 300 msec since the STA sends the probe frame when it does not receive the beacon frame within a predetermined time and to obtain the information on the APs from the response frame. After obtaining the information on the APs the STA determines whether or not to join the Basic Service Set (BSS). If the STA determines to join the BSS, the STA enters the authentication process.
Authentication is a process of determining whether the STA is what it is declared to be for security by communicating information between the STA and AP.
After passing the authentication process, the STA enters the association process. In the association process, the STA and the AP exchange information on the capability of the BSS, and a distribution system service (DSS) is triggered in order for the AP to communicate with another APs to exchange frames for the STA in their respective BSSs, forward frames to follow mobile stations as they move from one BSS to another, and exchange frames with a wired network. After the STA associates with an AP through the association process, the STA can transmit data.
FIG. 2a and FIG. 2b are message flowcharts for illustrating a conventional passive scanning-based handoff technique and a conventional active scanning-based handoff technique, respectively. As shown in FIG. 2a and FIG. 2b, the probe delays are main contributors to the entire handoff latency in both cases.
FIG. 3a and FIG. 3b are pie charts for illustrating percentages of delays occurring during the handoff procedure in the conventional passive scanning-based handoff technique and the active scanning based handoff technique, respectively. As shown in FIG. 3a and FIG. 3b, the probe delay is dominant in both the passive scanning-based handoff and active scanning-based handoff.
Thus, there is a need to reduce the probe delay for implementing seamless handoff in the WLAN.